Heated Mattresses

Just got back this past weekend from camping and it was a bit cold for my family so we used the heated mattresses. As we were breaking down the camper my wife noticed that on the plywood underneath the mattresses were our exact body shapes where we slept. Im assuming this is due to moisture and it will eventually go away. That being said I don't like that it does this so I'm going to get some sort of barrier to maybe stop this. We live in Florida so it won't be often we use them. Any thoughts